1. Field of the Invention
The present invention relates to a manufacturing method for a single crystal of calcium fluoride and a processing method for calcium fluoride powder.
2. Discussion of the Related Art
In the past, single crystals of calcium fluoride (CaF.sub.2, or calcium fluoride) have been manufactured primarily using a Bridgeman method (sometimes called a Stockburger method or a crucible lowering method).
The material for making a single calcium fluoride crystal for use within the visible or infrared range has typically been manufactured using a mixture of a set amount of crushed natural calcium fluoride or synthetic calcium fluoride (manufactured by using natural calcium fluoride) and a scavenger. However, for the purpose of manufacturing (i.e., growing) a single calcium fluoride crystal for use in the ultraviolet or vacuum ultraviolet range, the above materials cannot be used because crushed natural calcium fluoride or synthetic calcium fluoride absorbs in the ultraviolet or vacuum ultraviolet. Therefore, a mixture of a high purity powder of a chemically synthesized calcium fluoride and a scavenger has been used. However, because there is a significant reduction in volume when the powder is directly fused due to its high specific gravity, a cullet, which is a lump of material obtained by fusing the high purity powder once, is crushed and the resultant material, is sometimes used as a starting material.
The starting material is filled in a growth crucible, placed in a growth furnace, and maintained in a vacuum. Then, the temperature inside the growth furnace is gradually increased and the material is fused; followed by lowering the growth crucible, such that the material crystallizes from the bottom of the crucible and a single calcium fluoride crystal is grown.
When a single calcium fluoride crystal is manufactured as above from a powdered calcium fluoride using the Bridgeman method, a lot of gas impurities are adsorbed on the surface of the powder particles, and there is a significant reduction in the degree of the vacuum due to the desorption of the adsorbed gas during the increase in temperature. Therefore, it is impossible to heat the powder continuously or to increase the temperature at a specific rate up to the maintenance temperature. Consequently, heating at low power and vacuum exhaustion must be conducted intermittently. As a result, the de-airing (degassing) process using a single crystal manufacturing furnace takes an extremely long time, resulting in reduced productivity and delays in the manufacturing process.
When the powder is melted and crystallized, the volume is reduced to approximately 1/3 of the powdered state. Therefore, in general, a preprocessed product, such as fused glass or a cullet, is used as the material and filled in a growth crucible, and then a single crystal of calcium fluoride is grown. However, the desorbed gas from the powdered material is mixed in with the preprocessed product, and as with the single crystal manufacturing furnace, productivity is reduced when using the preprocess furnace.